First, have students derive the formula for the circumradius of an equilateral triangle, with sides of length \(s.\) The correct formula is \(s / \sqrt{3}.\) After that, have students derive the formula for the circumradius of an isosceles triangle, with base \(b,\) height \(h,\) and slant height \(s.\) The correct formula is \(r = s^2 / (2h).\) Its derivation can be seen here.

Conclude by giving your students these challenges:

- Fractions Jigsaw by NRICH
- 3388 by NRICH
- Olympic Logic by NRICH
- 2007 AMC 8, Problem 19

Cut Nets by NRICH: Have students pair the pieces by looking at the image. Then have students cut out the shapes, and tape them together, to see if they were right. If you allow students to cut out the shapes before pairing them, they will inevitably find pairs by trial-and-error, which is not the objective.

TODO: It would be nice if we had a larger version so it was easier to cut out the figures. It would also be nice if said image was in black-and-white, so that it was cheaper to print.